The present invention relates to a method for resin-coating optical transmission glass fibers.
Optical transmission glass fibers (hereinafter referred to as "optical fibers") have excellent transmission characteristics of high capacity and low loss. Furthermore, a transmission line using a optical fiber has no electrical inductance. Optical fibers are also light in weight. Due to these and other advantages, transmission systems utilizing optical fibers have been rapidly developed and put to practical use.
Since optical fibers are made of glass and can have a diameter as small as 200 .mu.m or less, mechanical strength is a very significant consideration. In order to improve the mechanical strength of an optical fiber cable, a tandem primary coating method has been employed in which an optical fiber base material is melted using a resistance furnace, a high-frequency induction furnace, a CO.sub.2 laser, an oxyhydrogen flame, etc. The optical fiber is thereafter drawn, and a resinous composition immediately coated onto the bare glass fiber and hardened. This retains the original strength of the glass and prevents the optical fiber from being damaged (see Japanese Published patent application No. 100734/76).
In applying the resinous composition coating, a die dipping method, a felt coating method, a spraying method, and the like have been used. Each method has specific advantages and disadvantages. Of these methods, the die dipping method and the spraying method are superior in that the resinous composition can be coated onto the fiber without contacting the fiber. Therefore, these two methods are used most widely. In particular, the spraying method is most suitable for high-speed coating because glass fibers while drawing at a high speed are subject to less influence due to temperature. The influence of temperature is a serious problem with the die dipping method. Furthermore, a coating film having a thickness of 2-3 .mu.m or less can only be obtained by employing the spraying method. On the other hand, the spraying method is disadvantageous in that the thickness of the coating film obtained by a single coating is limited, and also it is difficult to control the thickness thereof.
A conventional apparatus used in practicing the spraying method will hereinafter be explained.
In FIG. 1, an optical fiber 2 resulting from melt-drawing of a preform 1 is drawn downwardly and vertically. A resinous composition is coated onto the optical fiber 1 by a sprayer 3. The sprayer 3 may, for instance, be a spraygun such as the Model HP-PC102 produced by Olympus Co., Ltd. of Japan.
In order to recover the excess sprayed resinous composition and to prevent it from contaminating the surrounding area, spraying is usually performed in a spray coating vessel 4 which is closed except for holes through which the optical fiber passes. A feed unit 6 provides the sprayer 3 with the resinous composition as shown in FIG. 1.
The resinous composition is sprayed at a pressure of from 0.1 to 5 kg/cm.sup.2 by means of the sprayer 3. Because the spray coating vessel 4 is effectively closed, turbulence is produced and a good spray stream cannot be obtained. This tends to cause the optical fiber to swing out of position, making it impossible to uniformly coat it with the resinous composition.